This invention relates to a method and apparatus for measuring the permeability of a rock fracture to two or more immiscible fluids. More particularly, this invention relates to a method and apparatus for measuring the permeability of a rock fracture to two flowing phases such as a liquid and a gas.
Data on the relative permeability of a rock fracture to two or more immiscible fluids, such as a liquid and a gas, is needed in predicting fluid flow in such processes as nuclear waste isolation, petroleum reservoir engineering, and exploitation of geothermal energy. When two phases occupy a porous medium or a fracture, in general one phase wets the surface better than the other. The non-wetting phase must, therefore, be at a higher pressure; and the difference in pressure is called the capillary pressure.
In measuring two-phase flow, the capillary pressure must be uniform throughout the flow field, or errors result which are difficult to compensate for. A method to achieve uniform capillary pressure, when measuring the permeability of porous bodies, is disclosed in Hassler U.S. Pat. No. 2,345,935, wherein the wetting phase is fed to and from the porous sample body through solid end blocks placed respectively against the opposite ends of the porous sample body. A circular distribution groove, intersecting a bore passing through the end block, is formed in the respective face of each end block facing the porous sample body, and porous material is respectively placed in each of these circular distribution grooves so that the wetting phase must pass through this porous material in the end blocks before reaching the face of the porous sample body. The non-wetting phase (e.g., a gas) is fed directly to the face of the porous sample body through a bore which passes through the end block to the face of the porous sample body.
A somewhat similar system for measuring the relative permeability of fluids in a porous body is described in Rose U.S. Pat. No. 4,506,542, wherein the end blocks, however, are made of porous material, and the distribution grooves of Hassler are eliminated, with the wetting phase passing directly through the porous end blocks to and from the respective faces of the porous sample body.
However, while the distribution grooves containing porous material in the end blocks of Hassler and the porous end blocks of Rose do provide some distribution of the wetting phase into the porous body to be measured, the use of a single port in the end block of either Rose or Hassler to deliver the non-wetting phase to the face of the porous body leaves something to be desired, particular when one attempts to measure the permeability of a fracture in a rock structure, rather than the permeability of a porous body wherein the porous nature of the body being studied may itself provide a certain degree of distribution means for the non-wetting phase.
Osaba et al., in an article entitled "Laboratory Measurements of Relative Permeability", published in Petroleum Transactions, AIME, Volume 192 (1951), at pages 47-56, describe the measurement of the relative permeabilities to oil and gas on small core samples of reservoir rock and illustrate apparatus for making measurements by the Hassler method using semi-permeable discs at each end of the core sample to allow oil, but not gas, to pass. Provision is made for gas to enter and leave the core sample through radial grooves in the respective faces of the semipermeable discs.
In the study of porous materials, the distribution of the wetting and non-wetting phases to the face of the porous sample is not critical, because the three-dimensional network of pores within the sample ensures that essentially all pores have access to either phase, and will be occupied by one phase or the other as determined by capillary pressure. For a fracture, however, there is no such three-dimensional network of pores to accomplish the detailed distribution of the phases, so the uniform delivery of both the wetting and non-wetting phases to all parts of the fracture edge is necessary to obtain accurate results.
Measurement of fluid flow through a rock fracture has been previously measured. Jones et al. U.S. Pat. No. 4,884,438 describes the measurement of flow of a single phase through a fracture using inlet and outlet blocks which respectively contain inlet and outlet ports for transporting the fluid to and from the fracture face.
The determination of fluid saturation within a fracture is taught in Jones et al. U.S. Pat. No. 4,907,442, by establishing a functional relationship between fluid saturation and electrical capacitance for a multiple component fluid and then, after introducing the fluid between the fracture faces of the fractured media, the fluid saturation is determined from electrical capacitance measurements.
However, it would be desirable to provide a reliable method and apparatus for measuring multiple phase flow in a single fracture wherein both wetting and non-wetting phase are delivered to, and uniformly distributed across, opposite edges of a fracture in a manner which will permit accurate determination of the flow rates and pressure drops of the respective phases through the fracture to determine the permeability of the fracture, and measurement of the capillary pressure at the inlet and outlet edges of the fracture.
In a paper entitled "Two-Phase Flow Visualization and Relative Permeability Measurements in Transparent Replicas of Rough-Walled Rock Fractures", published by Lawrence Berkeley Laboratory as LBL-30161, and the contents of which were distributed in a printed publication in January, 1991, we discussed the use of porous end blocks placed against opposite edges of a rock fracture for delivery of a wetting phase to the rock fracture and grooves alternate with flats on the face of the porous block facing the fracture edge to deliver a non-wetting phase to the fracture edge.